Thick anodic coatings, commonly referred to as "hard anodize", are used to protect aluminum equipment against abrasion and corrosion. Definitive measurements of the thermal conductivity of this type of coating were not available in the open literature, even though hard anodic coatings have been in use for many years. Since anodic coatings on aluminum consist mainly of aluminum oxide and the thermal conductivity of bulk, crystalline aluminum oxide is very high, as high as that of some metals, the thermal conductivity of anodic coatings is widely believed in the industry to be quite high. In tests of very high-efficiency heat-transfer equipment, such as that used in underwater vehicle propulsion systems, it has become apparent that the thermal conductivity of the available commercial coatings is low enough to be a significant factor in the design of these systems. The thermal conductivity of typical commercial coatings was measured and found to be approximately 0.7 Watts/meter/C. On the other hand, the thermal conductivity of bulk, crystalline aluminum oxide is about 33 Watt/meter/C, or about a factor of 50 greater than that of hard, anodic coatings. This apparent discrepancy can be explained by the fact that anodic coatings, as they are commercially applied, are quasiamorphous materials (not crystalline) and that they contain large amounts of materials other than aluminum oxide. As a general rule, the thermal conductivity of amorphous materials is an order of magnitude or more smaller than that of the same material in crystalline form, so that the low thermal conductivity of anodic coatings is not very surprising despite the widely held belief to the contrary.
An anodic coating with enhanced thermal conductivity is needed for high performance systems which also require very good abrasion and corrosion resistance. Such systems would be those which require very efficient heat transfer due to limitations of size of the heat transfer area or the temperature difference over which the system is constrained to operate, and are exposed to large amounts of handling or service in a corrosive environment. Another use would be in certain electronic circuits, where aluminum is used as a heat sink, and when good dielectric strength as well as good thermal conductivity are necessary. Some applications concerning electronics require a thick, anodic coating for electrical insulation, but also require good thermal conductivity for dissipation of heat.
Typically, the anodization of aluminum and its alloys is an electrochemical process for producing a tough, electrically insulating coating on aluminum parts. The part to be anodized is immersed in an electrolyte, and a positive potential is applied to the aluminum part in reference to a cathode of lead or other material. As current flows between the aluminum anode and the cathode, aluminum on the surface of the anode is converted to aluminum oxide and a coating is formed. The coating also contains significant amounts of hydrated aluminum oxide in addition to anhydrous aluminum oxide, and anion from the electrolyte, e.g. the coatings contain about 14 percent sulfate for sulfuric acid-based electrolytes. The chemical composition and morphological properties of the coating depend greatly on the anodization parameters, such as voltage, current density, temperature, type and concentration of the electrolyte.
When a weak electrolyte is used, which will not dissolve the coatings, a thin non-porous coating is produced. The coating grows to a certain thickness and, since it is a very good electrical insulator, blocks further current flow, stopping the anodization process. These barrier type coatings are typically less than 1 micrometer in thickness, are easily abraded from the surface, and are used, for example, as dielectrics in electrolytic condensers. The electrolytes which produce this type of anodic coating are generally aqueous solutions of a very weak acid such as boric acid.
When the anodic coating has considerable solubility in the electrolyte, then a thin, barrier layer is formed on the aluminum surface and a porous outer layer is formed on the side exposed to the electrolyte. The pores initiated at sites on the coating continuously progress into the barrier layer allowing more aluminum to be converted. If the electrolyte is not so aggressive that the rate of dissolution of the coating is as great as the rate of coating formation then thick coatings (as much as 200 micrometers in thickness) can be applied to the aluminum part. These thick, porous coating are called "hard anodic" coatings due to their very high abrasion resistance, and have been used for the protection of aluminum surfaces from abrasion and corrosion for about forty years. A relatively strong acid such as sulfuric acid is used in aqueous solution to produce this type of coating.
The anodization parameters are interdependent, so that a change in one parameter frequently results in a change in one or more other parameters. For example, at constant anodization current the voltage necessary to produce a coating is determined to a large degree by the solubility of the coating in the electrolyte. The coating solubility is determined by the strength of the acid used, the concentration of the solution, and the solution temperature. Because the anodic coating is a very good electrical insulator, the anodization voltage is impressed across the coating during anodization and not across the volume of the electrolyte. The anodization voltage effects such coating properties as the porosity, chemical composition, and the morphology or degree of crystallinity in the coating.
Electrolytes which operate at low voltages are favored in commercial processes in order to reduce electrical power costs. Sulfuric acid electrolytes, used in most commercial hard coating processes, operate typically at voltages from 15 to 130 Volts. As the coating is formed on the aluminum part, a progressively higher voltage is required at constant current. Anodization above 130 volts is not feasible in sulfuric acid, because runaway anodization, referred to as "burning", takes place and causes severe damage to the part. Anodic coatings are the preferred means of protection of aluminum articles against abrasion and corrosion and enormous quantities of aluminum are anodized annually in the United States. For very inexpensive articles produced in mass quantities, electrical power costs of anodizing can be an important factor in their manufacture. However, for expensive pieces of equipment, which must have long service lifetimes in severe environments, the electrical power cost will be outweighed and a coating with improved thermal conductivity is desirable for high-efficiency, heat-transfer applications.
A family of organic acids can be used in anodization electrolytes to produce thick coatings at voltages higher than 130 volts. This class of aqueous electrolytes based on the organic carboxylic acids was described by J. M. Kape in the Transactions of the Institute of Metal Finishing, Volume 45, 1967, pages 34-42. These acids are, in general, considerably weaker than sulfuric acid. Kape showed that thick anodic coatings can be produced at room temperature with electrolytes based on these acids or mixtures of these acids, which have comparable abrasion resistance to the hard anodic coatings produced in sulfuric acid solutions at much lower temperatures. Oxalic acid is the most commonly used member of this group of acids in anodization electrolytes. It and other organic carboxylic acids have been used commercially as additives in sulfuric acid-based electrolytes to raise the temperature at which hard anodizing can be performed in order to reduce refrigeration costs. They are not often used alone without a more aggressive acid, however, because anodization in aqueous solutions of these acids may require high voltages and thus higher electrical power cost.
Thus, a continuing needs exists in the state-of-the-art for an anodic coating with enhanced thermal conductivity for high performance systems which also require very good abrasion and corrosion resistance.